1. Field of the Invention
The invention relates to a combined mask-frame for a colour picture tube i.e. an arrangement with an approximately rectangular mask frame and a mask which is attached to the frame by means of spot welds.
2. Description of the Prior Art
Masks for colour picture tubes are drawn from special metal sheet which are provided with oblong holes. A mask skirt is formed during drawing and the mask surface is curved. The mask skirt is attached to the frame at eight or twelve spot welds by means of electric spot welding. As a rule, spot welds are placed adjacent to the short and long centre axes and at the corner points. With colour picture tubes with screen diagonals of more than approximately 20 inches, additional spot welds are placed on the long sides between the centre and the corners.
With respect to the properties of the joint between the mask and the frame, a high degree of rigidity is desirable to prevent the mask from being distorted by shocks and vibrating excessively as a result of sound waves such as those transmitted by a loudspeaker inside a television set. For other reasons a loose joint is desirable--especially because the mask and the frame have different expansion characteristics during tube manufacture and tube operation, especially at high cathode ray currents.
In order to meet these contradictory demands in the most effective way, the masks are provided with stiffening creases at certain points and with isolation slits at other points. Attention must be paid to the height of the spot welds in relation to the height of the mask skirt. Such general problems and a solution for them are described, e.g. in DE 31 15 799 A1.
This optimization problem, like all technical optimization tasks, is characterized by the desire to find an arrangement representing a compromise which is better than that offered by previous alternative solutions. In practical terms this means in the present case that for many years a solution has been sought to the problem of attaching a mask to its frame in such a way as to improve shock and vibration resistance while minimizing the effect of expansion due to heating on the colour display quality of the tube.